1. Technical Field of the Invention
The present invention relates to a signal transmission apparatus adapted for use in a system for transmitting an audio signal through a network while controlling audio amplifiers, mixers and other audio equipment connected to the network.
2. Prior Art
In a sound system used at a big concert hall and the like, multiple channels of audio signals generated by a mixing system and the like are sounded from many loudspeakers through many amplifiers. Since the number of cables will become enormous if a cable for audio signal transmission is installed for each individual channel, it is desirable to convert multiple channels of audio signals into audio data packets and to transmit them through a digital network.
The technology of CobraNet (trademark) is known as a protocol method of transmitting multiple channels of audio data in real time through a network of a CSMA/CD (Carrier Sense Multiple Access with Collision Detection) system such as the Ethernet (registered trademark). CobraNet technology is disclosed in Non-patent literature, Audio Networks An Overview, Cirrus Logic, Inc., 2001.1. In the CSMA/CD system, an arbitration is used when a collision occurs, that is, when two or more nodes start transmission simultaneously. However, the occurrence of a collision actually causes band losses due to arbitration. In view of this, CobraNet assigns a transmission period of audio data to each node involved in the network within one transmission cycle to avoid a collision, thus enabling efficient transmission of up to 128 channels of audio data.
Referring now to FIG. 3(a), a general outline of the protocol of CobraNet will be described. First, in the protocol of CobraNet, audio data is outputted from each node on the network within one transmission cycle (denoted 200 in the figure) set at an interval of 1.33 msec. Then, one of nodes is set as a special node (called the “conductor”) for managing the transmission cycle 200. At the beginning of each transmission cycle 200, the conductor outputs a beat packet (startup packet) 201 on a network 1000.
The output of this beat packet 201 stimulates all nodes including the conductor node to output audio data packets 211, 212, . . . , 21n, respectively, in predetermined order. These packets are called “bundles” and “one” bundle contains audio data of several channels, for example, a maximum of “8” channels. Each bundle is given a bundle number that does not overlap with other bundles. A node seeking to receive outputted audio data determines a target bundle from the bundle number and captures the bundle that contains audio data to be received, and retrieves audio data of a desired channel from the received bundle. Each of the packets 211, 212, . . . , 21n transmitted from each node may occasionally carry two or more bundles. Then, CobraNet provides a serial communication period in which a serial communication packet 220 can be transmitted, in an idle time interval within the transmission cycle 200, the idle time interval being provided after completion of output of all the packets within one transmission cycle 200.
Thus, CobraNet enables serial communication using the idle time interval provided within the transmission cycle 200. However, since the band for serial communication is narrow by definition, the transmission of control data causes a problem of increasing delay time. In addition, since the delay time depends on the number of bundles of audio data, it is difficult to stably control many audio amplifiers and other audio equipment involved in the network, and to stably collect status data of the amplifiers and other audio equipment from the network.
For such reasons, the serial communication packet 220 defined in the CobraNet protocol is not so often used in practice. Instead, QSControl (trademark) and Audia (trademark) are known for example as technology of transmitting control data to a system to which CobraNet is applied. In these technology, the control of each node, for example, the collection and control of the states of the amplifiers, is performed through a dedicated network for control data independent of the CobraNet network.
However, the use of the separate network independent of the CobraNet network to control the amplifiers and the like requires physical connection for both CobraNet network and the separate network. Stated otherwise, an audio data network cable and a control data network cable are separately required to connect each node. This increases the number of cables used and hence causes a problem of increasing the trouble in building up the sound system.